In hedge trimmers and similar applications having reciprocating cutting members it is common practice to impart reciprocal motion to the cutting member by means of a scotch yoke drive mechanism. Thus, the cutting member is provided with a yoke portion having a cam follower slot, and a circular cam, which is eccentrically mounted upon a drive member, engages the yoke to impart reciprocal motion to the cutting member upon rotation of the drive member. When two reciprocal shear bars are employed it has been proposed in the past to employ such a drive mechanism, and one such example is shown in U.S. Pat. No. 2,630,628 issued Mar. 18, 1953. In the drive shown in this patent two juxtapositioned cams engage cam follower slots into juxtapositioned yokes. However, the drive shown in this patent is not satisfactory as during operation of the cutting devices, forces are imposed upon the cutting members which tend to move the yokes out of their normal plane of operation along the axis of the drive member. If these forces are along the as one cam is passing the cam follower slot which is adapted to be engaged by the other cam it is possible for the first cam to interfere or engage with the other cam follower slot which may result in the parts locking and/or breaking. Repeated interference will eventually cause a breakdown of the parts.
In view of the deficiencies shown in the aforesaid patent design it has been proposed to separate the yokes so they are not in face-to-face contact. In accordance with one design the yokes are offset from the principal plane of the cutting members and are engaged by cams disposed on opposite sides of the driving pg,3 member. While the foregoing design is generally satisfactory it is somewhat costly, particularly since it is necessary to provide the cutting members with offset yokes. This is not considered desirable in the design of the cutting members as forces to the cutting members are transmitted from the cam follower slot in the yoke in shear through the offset portion requiring either reinforcement at the point of offset or requiring higher strengths and thicknesses of materials to be employed in the overall design of the cutting member. Alternatively it has been proposed to mount both of the cams on one side of a drive member, the cams being separated from each other by a spacing shim which extends between the yokes of the driven members. While this design is generally satisfactory, it is desirable to simplify the design by eliminating the requirements of a spacing shim.